A conviction for DWI 2d has jail time as a requirement for probation, so he's probably looking at some jail time. He needs to find out about Dallas County DWI court that might allow him to avoid the jail time and get him some much needed alcohol counseling.
Jail time is a real possibility. His only hope at avoiding jail time would be to hire an experienced DWI attorney and to try to either win the case or convince the DA to drop the charge down to a DWI 1st. If his probation is still unresolved he faces even more jail time as a result of the probation revocation as well.
He needs to talk to his local counsel and take his advice. There are lots of details about his probation, and the second DWI that are going to go into determining how this all pans out. Make sure you have an attorney you trust, and who has experience with handling DWIs and listen to them.
You ask if your brother will do any jail time for a second offense DWI, considering that he is still "technically" on probation on his first DWI - because he hasn't completed his community service*.
The answer depends on several factors, not the least of which are: 1) Is he still on probation (technically or otherwise), 2) Can you beat the new DWI charge? Those are the first questions which need to be addressed. I'm not sure what you mean by him being "unsatisfactorily discharged from probation". ANY discharge from probation is satisfactory in most circumstances.
If he's still on probation, jail time is a distinct possibility. If he gets probation on the new case, there is 72 hours county jail time required as a condition of that probation, but there are some alternative sentencing options that can be explored.
Fortunately, Dallas is generally pretty good about not putting people in jail under these circumstances. Most Dallas judges take the approach that jail time does little or nothing to rehabilitate DWI offenders, and is a waste of taxpayer's money, for the most part. If this was in Denton county, just to the north, your brother would likely be spending three to six months in jail. In any event, your brother needs a good attorney!
Best of luck to you both!
PS - *Your brother apparently isn't very smart, but I'd be happy to represent him to the best of my ability anyway.
This answer is for general purposes only, and is no subsitute for specific legal advice that would come from an attorney hired by you, having full knowledge regarding the facts and circumstances of your individual situation. This correspondence does not create an attorney-client relationship, and you should consult with one or more attorneys prior to acting on any of the information provided in this response.